1. Field of the Invention
The present invention relates to compound semiconductor devices and more particularly, to formation of a contact to an n type Al.sub.x Ga.sub.1-x As (0.ltoreq..times..ltoreq.1) material.
2. Description of the Related Art
GaAs has been considered to offer a very promising prospect as a material for a high speed device, because the material has a mobility of about 6 times larger than that of silicon (Si) and can be easily made into a semi-insulating substrate.
Al.sub.x Ga.sub.1-x As obtained by adding aluminum (Al) to GaAs is very close in lattice constant to GaAs and allows good epitaxial growth with GaAs. For this reason, attention has been increasingly directed to development of heterojunction devices based on an Al.sub.x Ga.sub.1-x As/GaAs heterojunction.
As a result, many sorts of devices have been developed, including, for example, an Al.sub.x Ga.sub.1-x As/GaAs heterojunction bipolar transistor (HBT) having a wide gap emitter of Al.sub.x Ga.sub.1-x As, and a high electron mobility transistor (HEMT) having an electron supply layer of Al.sub.x Ga.sub.1-x As.
For the purpose of enhancing the performances of these devices, it is very important to reduce the resistance of their ohmic contact.
Generally speaking, it is difficult to form an ohmic contact of low resistance on an Al.sub.x Ga.sub.1-x As layer (0&lt;.times..ltoreq.1). For this reason, even when such an Al.sub.x Ga.sub.1-x As layer must be placed as the top layer of a device, an n.sup.+ type GaAs layer has been formed on the Al.sub.x Ga.sub.1-x As layer as a cap layer for the ohmic contact. To this end, the composition of metal material of its electrodes, the temperature of heat treatment for the formation of the alloy, etc., have been correspondingly devised.
Since these GaAs and Al.sub.x Ga.sub.1-x As materials are large in band gap and low in the upper limit of obtained electron concentration when compared with Si material, the reduction of the ohmic contact resistance is limited thereby.
To overcome this, there has been proposed a method of using In.sub.x Ga.sub.1-x As material having a band gap smaller than the Al.sub.x Ga.sub.1-x As or GaAs as a contact layer (refer to J. Vac. Sci. Technol., 19 (3), 1981, pp626-627).
FIG. 5 shows, as one of such prior art examples, an ideal energy band between an n type GaAs layer and a metallic electrode with an n.sup.+ type In.sub.x Ga.sub.1-x As layer and an n.sup.+ InAs layer interposed therebetween at the time of forming a contact to the n type GaAs. For the purpose of smoothly connecting together the bands of the GaAs and InAs layers, the graded-composition layer of the In.sub.x Ga.sub.1-x As material (x=0.fwdarw.1) is inserted between the GaAs and InAs layers. Since no Schottky barrier is present between the InAs layer and the electrode, a low resistance contact can be obtained.
However, there is a lattice misalignment as large as about 7% between the InAs and GaAs layers.
This lattice misalignment causes the InGaAs layer to be subjected to a misfit dislocation. In the event where the thickness of the InGaAs layer is below its critical thickness, no misfit dislocation takes place. When it is desired to form an ohmic contact of lower resistance, however, it is preferable that an In mixed crystal ratio x is closer to 1 and the critical thickness becomes correspondingly smaller. When the InGaAs layer has a mixed crystal ratio x of 0.5, its critical thickness is below several ten .ANG.. In addition, as the mixed crystal ratio x is closer to 1, the thickness of the In.sub.x Ga.sub.1-x As graded-composition layer necessary for smoothly connecting the bands of the InAs and GaAs layers is larger. Meanwhile, when the element performance, process, etc., are taken into consideration, it is preferable to make small the thickness of the contact layer. From these reasons, when an ohmic contact of low resistance is to be formed with use of a practical structure, the occurrence of a misfit dislocation cannot be avoided.
Furthermore, the occurrence of such a misfit dislocation is concentrated, in particular, in the In.sub.x Ga.sub.1-x As as an intermediate layer to compensate for the carriers in this area, which involves a problem that the carrier concentration is reduced.
It is known that the upper limit of the usual carrier concentration, which depends on the epitaxial growth conditions and so on, is about 2.times.10.sup.19 cm.sup.-3 and about 1.times.10.sup.19 cm.sup.-3 for InAs and GaAs respectively. In the case of the InAs/In.sub.x Ga.sub.1-x As/GaAs structure, a large carrier concentration dip occurs, in particular, in the intermediate layer of In.sub.x Ga.sub.1-x As. When the thickness of the In.sub.x Ga.sub.1-x As is made sufficiently large, the dislocation density (cm.sup.-3) is also reduced and the carrier concentration dip is correspondingly decreased.
When consideration is paid to the application of such a structure to a semiconductor device having an n type ohmic contact, however, it is not practical to increase the layer thickness.
Turning now to FIG. 6, there is shown a measurement result of carrier concentration distribution in the InAs/In.sub.x Ga.sub.1-x As/GaAs structure doped with silicon. In the drawing, the film thickness of the In.sub.x Ga.sub.1-x As layer was set to be 500.ANG. and 1.5.times.10.sup.19 cm.sup.-3 of silicon was doped into the InAs and In.sub.x Ga.sub.1-x As. As will be seen from the drawing, the carrier concentration largely drops in the In.sub.x Ga.sub.1-x As layer and there is an area therein where the carrier concentration is substantially zero especially in the vicinity of the interface with the GaAs layer. Under such a condition, the contact resistance when a current x flows in a vertical direction with respect to the semiconductor layer was as high as 5.times.10.sup.-6 .OMEGA. cm.sup.2.
Thus, the conventional structure has had such a problem that, even when a low-resistance contact can be obtained at the InAs/electrode interface, it is impossible for the entire contact to have a sufficiently low resistance.
In this way, the conventional method of forming the ohmic contact to the n type GaAs layer using the In.sub.x Ga.sub.1-x As layer has had a problem that the carrier compensation caused by the misfit dislocation causes the reduction of the carrier concentration, which results in that an area having a high resistance is formed and the contact resistance becomes correspondingly large.